Wave Energy Converter, and Associated Operating Method and Control Device

ABSTRACT

A wave energy converter comprises a rotor, which is designed to convert a wave movement of a body of water, in which the wave energy converter is arranged, into a rotational movement of the rotor, wherein a mean density of at least one rotating component of the rotor can be adjusted and/or is lower than the mean density of the fluid surrounding the rotor in the body of water. The disclosure likewise relates to a corresponding control device and an operating method.

This application claims priority under 35 U.S.C. §119 to patentapplication no. DE 10 2012 012 055.9, filed on Jun. 19, 2012 in Germany,the disclosure of which is incorporated herein by reference in itsentirety.

The disclosure relates to a wave energy converter comprising at leastone rotor, which is designed to convert a wave movement into arotational movement of the at least one rotor, to a method for operatinga wave energy converter of this type, and also to a correspondingcontrol device.

BACKGROUND

A range of different apparatuses are known for the conversion of energyfrom water movements in bodies of water into usable energy. For example,G. Boyle, “Renewable Energy”, second edition, Oxford University Press,Oxford 2004 provides an overview of this. Apparatuses of this type arereferred to within the scope of this application as “wave energyconverters”.

In wave energy converters, the energy can be extracted from the watermovement in different ways. For example, buoys or floats floating on thewater surface are known and drive a linear generator as a result oftheir rising and falling movements. In another machine concept, a planarresistance element is positioned on the seabed and is tilted back andforth by the movement of the water. The kinetic energy is converted in agenerator into electrical energy for example.

Wave energy converters that are arranged with their moved parts belowthe water surface and that utilize a wave orbital movement present thereare of particular interest within the scope of the present disclosure.

The wave orbital movement can be converted by means of rotors into arotational movement. For this purpose, rotors with coupling members, forexample in the form of lift profiles and/or Flettner rotors, can beused. A system of this type is disclosed in US 2010/0150716 A1.

The rotor with its coupling members is advantageously to be largelywave-synchronous, that is to say it is to orbit with a mean speed ofrotation that corresponds to the wave orbital movement or isproportional thereto. If, for example, the rotational frequency of therotor corresponds to the wave frequency, largely static incident flowconditions are produced at the coupling members and lead to a largelycontinuous torque at the rotor shaft. This can be fed directly into agenerator. Excessive mechanical stresses and/or nonuniformities in theoutput power of the wave energy converter can thus be avoided.

In particular in the open sea, rather different wave states occurhowever. Bedsides what are known as deep seas, in which the waves occurvery regularly, these wave states also include wave states in which thewave characteristic changes continuously as a result of thesuperimposition of different waves. Within the scope of thisapplication, the first wave state is referred to as a “monochromatic”wave state, and the second wave state is referred to as a“multichromatic” wave state. Completely monochromatic wave states occurrarely in nature, and therefore the term “monochromatic” also includeswaves that have a certain, albeit low, multichromatic component.

Although the wave states generally do not change suddenly and inaddition can be relatively well predicted, the speed of rotation of acorresponding rotor often cannot be adapted quickly enough in practice.This is true in particular for multichromatic wave states.

The disclosure therefore attempts to create a possibility of improvingthe synchronizability of a wave energy converter, even in the case ofmultichromatic wave states.

SUMMARY

In accordance with the disclosure, a wave energy converter comprising atleast one rotor, which is designed to convert a wave movement into arotational movement of the at least one rotor, a method for operating awave energy converter of this type and also a corresponding controldevice having the features described herein are proposed. Advantageousembodiments are disclosed in the following description.

The present disclosure is based on the finding that a high rotor momentof inertia of a wave energy converter is advantageous for (largely)monochromatic wave states, since said moment of inertia has astabilizing effect on the rotor. For multichromatic wave states however,a minimal rotor moment of inertia appears to be advantageous, since inthis case the speed of rotation has to be adapted frequently.

In the ideal case, the variation of the speed of rotation is to beachieved merely by a corresponding variation of the generator torque,that is to say of a (braking) counter torque to the rotor torqueproduced by the wave movement. If the rotor moment of inertia is high,the necessary acceleration of the rotor may, in some circumstances, notbe achieved merely by a reduction of the generator torque however (andtherefore by a correspondingly predominant contribution of the (driving)rotor torque). In this case, the rotor would have to be drivenadditionally by the generator, which is disadvantageous in terms ofenergy, in particular with high inertia.

Previously, it was assumed that a largely neutral hydrostatic lift ofthe rotor components in the water was particularly advantageous. Thiswas considered to be important in particular for rotors of which themovable components are not distributed uniformly about the axis ofrotation, since in this case preferred positions would otherwise becreated, which could disturb the operation. In particular withrelatively large rotor diameters, for example from 10 m to 30 m, andwith the use of relatively large lift profiles having spans in theregion of more than 10 m and a chord length from 1 m to 8 m,considerable rotor moments of inertia are produced however with neutralhydrostatic lift, inter alia due to the relatively large rotatingmasses, and oppose efficient operation with regular adaptation of thespeed of rotation in multichromatic wave states. In addition,hydrostatic lift members of not insignificant size are required inprevious systems in order to tension the rotor beneath the water surfacewith respect to an anchoring system and thus support the occurringtorque for example.

Contrary to the aforementioned previous concept, the disclosure proposesthe use of rotor components, in particular of lift profiles or ofFlettner rotors, lever arms and the like, which have a lower meandensity than the surrounding water, for example seawater. The rotormoment of inertia compared to rotors having components with neutral liftand/or greater mean density than that of the surrounding fluid is thusreduced accordingly, which leads to a much greater adjustment dynamicand therefore adaptability of the rotor to changing, in particularmultichromatic, wave states.

In addition, the optimized components provided in this way function ashydrostatic lift members, which reduces the need for additionalhydrostatic lift members and therefore saves system costs. In addition,the machine is smaller on the whole due to the saving of furtherhydrostatic lift elements and is therefore more transparent in terms ofhydrodynamics.

In accordance with the disclosure, a rotor for a rotating wave powerplant is therefore provided, in which the lift profiles and/or othercomponents have a lower mean density, at least occasionally, compared toseawater. Here, the components that are arranged at a large radialdistance from the rotor axis are formed accordingly in particular.

A lower mean density can be achieved for example by means of ashell-like structure, supported by struts where necessary. The enclosedair volumes lead to a considerable reduction of the mean density.Alternatively or additionally, corresponding cavities can also be filledwith foam, preferably with a closed-pore foam, in order to be protectedagainst infiltrating water.

To achieve a low mean density, fiber-reinforced plastics can alsoadvantageously be used. In this case, the lift profiles can befabricated completely from these materials, as is known for example fromthe field of wind energy. Due to the high mechanical loads occurring inparticular in the field of wave energy, combinations of a supportingframe, for example formed from metal struts, with a covering formedpreferably from fiber-reinforced plastics or other materials, such astextile fabrics, are also advantageous however.

With use in the rotor of components having a reduced mean densitycompared to water, a largely symmetrical rotor is preferably used. Here,“symmetry” is to be understood to mean that the corresponding componentsare distributed largely uniformly about the axis of rotation of therotor. Due to a symmetry of this type, a preferred position of the rotoris avoided because the center of mass and center of lift coincide withthe axis of rotation. This prerequisite is largely met for example inthe case of a rotor having two lift profiles offset by 180° or threelift profiles offset by 120°, etc. Other arrangements may also beadvantageous.

If, by contrast, the rotor were to have only one lift profile of lowerdensity, the rotor rotation would be superimposed by periodic forces asa result of the hydrostatic lift and/or weight of said rotor. This maylead to a failure of the rotor, specifically if, during the downwardsmovement, the hydrostatic lift forces are greater than the forcesproduced by the hydrodynamic lift at this one lift profile.

Deviations from the symmetry requirement are possible within a certainscope and can be compensated for by a control/regulation unit of acorresponding wave energy converter. Deviations of this type are causedin particular as a result of the fact that different lift profiles of arotor are often arranged at different angles of attack in relation to arotor tangent (are “asymmetrically pitched”). In addition, the geometryof the wings can deviate from one another due to geometricaltransformations for adaptation to a circular motion. To compensate fordifferences of this type at least within a certain scope, equalizationmasses, lift members and/or equalization members can be advantageouslyused and positioned on the rotor or the lift profiles thereof.

In a particularly advantageous embodiment, the mean density of the rotoror a corresponding component can be changed during operation. Asexplained previously, a high rotor moment of inertia can be advantageousfor a more monochromatic wave state, whereas a substantiallymultichromatic operation is optimized by a reduction of the rotor momentof inertia. The mean density can be changed to a significant extent bypumping seawater into and out from cavities in the rotor components (inparticular in the coupling members) or by pumping seawater aroundbetween components. Here, corresponding cavities may also be formed aschambers for example. The chambers provided in this case may also befilled with foam in part and/or flooded or bailed individually by meansof corresponding devices, as is known in the case of aircraft tanks. Thedensity of seawater (depending on salt content) is between 1020 and 1030kg/m³. The elements in question therefore have a correspondingly lowerdensity, which can also be adapted to the respective salinity. Sincewave states do not change suddenly, it appears to be advantageous, withthe presence of a largely monochromatic wave state, to increase therotor moment of inertia. In the event of the occurrence ofmultichromatic waves, the moment of inertia can be reduced again bypumping out seawater. Seawater is preferably pumped in and out bysuitable filters in order to protect the interior of the rotorcomponents against fouling or soiling. The increased hydrostatic liftproduced by pumping completely empty the components, for example thelift profiles, can also be used to bring the wave energy converter tothe water surface, for example for repairs or maintenance.

Due to the proposed change of the rotor moment of inertia, thehydrostatic lift of the rotor is naturally also changed. This change canbe compensated for during operation advantageously by means of acorresponding adaptation of further hydrostatic lift members.

A particularly advantageous embodiment further comprises means, forexample in the form of corresponding lines and/or pumps, which aredesigned to pump fluid back and forth between components of the waveenergy converter, in particular between rotating and static components,in a selective and metered manner. The total mass or the cumulative liftadvantageously is not changed hereby. No foreign bodies, contaminations,animals, or the like are introduced into the machine. At the same timehowever, this allows the influence on inertia according to thedisclosure.

Reference is made to the explanations above and below with regard tofeatures and advantages of the method likewise proposed in accordancewith the disclosure.

The disclosure and preferred embodiments will be explained in greaterdetail hereinafter with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of velocities and accelerations ofwater particles below the surface of a body of water moved in a wavedmanner.

FIG. 2A shows a schematic illustration of wave orbital movements belowthe surface of a body of water moved in a waved manner in shallow water.

FIG. 2B shows a schematic illustration of wave orbital movements belowthe surface of a body of water moved in a waved manner in deep water.

FIG. 3 a shows a wave graph of an amplitude plotted in relation to awave frequency in Hz for a monochromatic wave state in a body of watermoved in a waved manner.

FIG. 3 b shows a wave graph of an amplitude plotted in relation to awave frequency in Hz for a multichromatic wave state in a body of watermoved in a waved manner.

FIG. 3 c shows a wave graph of a wave rise or fall in m for a fixedposition plotted in relation to a time in s for a monochromatic wavestate in a body of water moved in a waved manner.

FIG. 3 d shows a wave graph of a wave rise or fall in m for a fixedposition plotted in relation to a time in s for a multichromatic wavestate in a body of water moved in a waved manner.

FIG. 4 shows a schematic illustration of a wave energy converter inaccordance with an embodiment of the disclosure in a body of water movedin a waved manner.

FIG. 5 shows a schematic illustration of resultant forces acting on thewave energy converter according to FIG. 4.

In the figures, like or functionally like elements are denoted byidentical reference signs. A repeated explanation is not provided.

DETAILED DESCRIPTION

FIG. 1 shows (local) velocities {right arrow over (v)} and accelerations{right arrow over (a)} of water particles below the surface of a body ofwater moved in a waved manner, for example in the sea, in the form ofhorizontal components u and vertical components w of the local watermovement and also of the associated horizontal and verticalaccelerations a_(x) and a_(z). In this case, the radial or orbital pathsR are provided, which, as explained in FIG. 2, in deep water arecircular and in shallow water increasingly adopt the shape of ellipses.The wave propagates in a direction of propagation p and is assumed in asimplified manner to be sinusoidal.

The coordinate system specified in FIG. 1 is also used in FIG. 2. Thecoordinate axis x corresponds to the direction of wave propagationparallel to the water surface, and the coordinate axis z corresponds tothe vertical to the water surface. The coordinate axis y runs parallelto the water surface and perpendicular to the direction of wavepropagation.

At a wave crest (0. π), all the water particles move in the direction ofwave propagation p (there is thus only a horizontal movement component upresent), and at a wave trough (1. π), said water particles move againstthis direction (referred to hereinafter as −u). Here, the verticalmovement components are zero. At π/2 and 3π/2, that is to say the zerocrossings of a corresponding wave function, there is no horizontalmovement; the water particles move exclusively upwards (movementcomponent w) or downwards (movement component −w). The movement may besuperimposed however by a current.

FIG. 2 shows the radial or orbital paths R in shallow water in sub-FIG.2A and in deep water in sub-FIG. 2B. In shallow water (sub-FIG. 2A), thehorizontal components u exceed the vertical components w of the localwater movement in terms of amount. The horizontal extent (2·A) of theorbital paths R is always greater than the vertical extent (2·B)thereof, and therefore A≠B. At the bottom of the body of water, thevertical component w of the local water movement is zero, that is to saythere is only still a horizontal movement present.

By contrast, in deep water (sub-FIG. 2B), A=B. The radii reduce howeverstarting from the mean water surface S to the bottom B of the body ofwater.

Wave graphs of monochromatic and multichromatic wave states in a body ofwater moved in a waved manner are illustrated in FIGS. 3 a-3 d. In thegraphs depicted in FIGS. 3 a and 3 b, an amplitude is plotted on theordinate in relation to a wave frequency in Hz on the abscissa. In thegraphs depicted in FIGS. 3 c and 3 d, a wave rise or fall in m for afixed position (“sea condition”) is plotted on the ordinate in relationto a time in s on the abscissa.

In the graphs of FIGS. 3 a and 3 c, a largely monochromatic wave stateis illustrated, whereas the graphs of FIGS. 3 b and 3 d by contrastillustrate a multichromatic wave state, which is produced from thesuperimposition of two main frequencies and also an addition of furtherfrequency components.

For the case illustrated in the graphs of FIGS. 3 a and 3 c with justlow fluctuations of the period, it can be seen that, with accordinglyadapted (specifically slightly fluctuating) rotational speed of a rotor,largely constant flow conditions can be set at the lift profiles.

For the case illustrated in the graphs of FIGS. 3 b and 3 d, this is notto be expected. Due to the complexity of the wave state, there is inthis case the need to change the speed of rotation of the rotor largelycontinuously in order to produce at the wings incident flow conditionsthat generate at the rotor a driving torque, which can be tapped andconverted by means of the generator.

As mentioned, a high rotor moment of inertia can be advantageous forlargely monochromatic wave states, since this has a stabilizing effecton the rotor. For multichromatic states however, a minimal rotor momentof inertia appears to be advantageous, since in this case the speed ofrotation has to be adapted frequently.

FIG. 4 illustrates a wave energy convertor 1, as may form the basis ofthe present disclosure, with a housing 7 and a rotor 2, 3, 4 mountedrotatably thereon with a rotor base 2 and two coupling members 3 eachfastened via lever arms 4 to the rotor base 2 in a rotationally engagedmanner. The rotor 2, 3, 4 is arranged below the water surface of a wavedbody of water, for example of an ocean. In this case, deep-waterconditions are present for example, under which the orbital paths of thewater molecules run largely in a circular manner. An axis of rotation ofthe rotor (perpendicular to the paper plane) is oriented largelyhorizontally and largely perpendicularly to the direction of propagationof the waves of the waved body of water. The coupling members 3 areformed as hydrodynamic lift profiles in the example shown. At least someof the rotating components of the wave energy converter 1 may preferablyhave a lower mean density, at least occasionally, than the surroundingwater in order to minimize a moment of inertia, as explained. This istrue in particular for components of the rotor 2, 3, 4 distributeduniformly about the axis of rotation, for example the coupling members 3and/or the lever arms 4 in the case of the wave energy converter 1, suchthat there is no asymmetry in this regard and therefore no preferredposition.

The coupling members 3 are arranged at an angle of approximately 180°relative to one another. The coupling members 3 are preferably mountedin the vicinity of their center of pressure in order to reducerotational torques on the coupling members 3 occurring during operationand therefore so as to reduce the demands on the mount and/or theadjustment devices.

The radial distance between a fixing point of a coupling member 3 andthe rotor axis is 1 m to 50 m, preferably 2 m to 40 m, and morepreferably 6 m to 30 m. The chord length of the coupling members 3 is 1m to 8 m for example. The span (perpendicular to the paper plane) may be6 m or more for example. The wave energy converter is also suitable foruse with two-sided rotors or what are known as squirrel-cage rotors, inwhich coupling members 3 with corresponding or adapted spans are eachattached in the axial direction on both sides to a rotor base 2 orbetween two rotor bases 2 via the explained lever arms 4.

Two adjustment devices 5 for adjusting the angle of attack or pitchangle α_(P,1) and α_(P,2) of the two coupling members 3 relative to thetangent to the rotor are shown here running perpendicularly upwards anddownwards. The two angles of attack α_(P,1) and α_(P,2) are preferablyoriented in opposite directions and for example have values from −20° to+20°. In particular when starting the wave energy convertor 1, greaterangles of attack may also be provided however. The angles of attackα_(P,1) and α_(P,2) may preferably be adjusted independently of oneanother. The adjustment devices may be adjustment devices comprising anelectric motor for example (preferably with stepper motors) and/or maybe hydraulic and/or pneumatic components.

The two adjustment devices 5 may additionally each be assigned a sensor6 for determining the current angle of attack α_(P,1) and α_(P,2). Afurther sensor (not illustrated) can determine the angle of rotation ofthe rotor base 2 relative to the housing 7.

The wave energy converter disclosed herein is also suitable for systemswithout adjustment devices 5 for adjusting the angle of attack or pitchangle α_(P,1) and α_(P,2) and/or corresponding sensors and/or for waveenergy convertors based on different operating principles, such asFlettner rotors. As is known, Flettner rotors are drives in the form ofa rotating cylinder that is exposed to a flow and that generates a forcetransversely to the incident flow by means of the Magnus effect. Forexample, cylinders formed from sheet metal arranged normal or horizontaland perpendicular to the direction of flow and rotatable by means of adrive and preferably having end discs are concerned in this case. Thegenerated force can be influenced by influencing the speed of rotation.

The orbital flow flows against the wave energy converter 1 at anincident flow velocity {right arrow over (v)}(t). The incident flow hereis the orbital flow of sea waves, of which the direction changescontinuously with an angular velocity Ω. In the illustrated case, therotation of the orbital flow is oriented in an anti-clockwise direction,therefore the respective wave propagates from right to left. In themonochromatic case, the incident flow direction changes here with theangular velocity Ω=2πf=const., wherein f is the frequency of themonochromatic wave. In multichromatic waves, Ω by contrast undergoes achange over time Ω=f(t), since the frequency f is a function of time,f=f(t). The rotor 2, 3, 4 rotates synchronously with respect to theorbital flow of the wave movement with an angular velocity ω₁, whereinthe term “synchronicity” is to be understood as an average over time. Inthis case, Ω≈ω₁ for example. A value or a value range for an angularvelocity ω₁ of the rotor is thus predefined on the basis of an angularvelocity Ω of the orbital flow or is adapted thereto. In this case, atemporary or short-term adaptation (for example with a superimposedangular velocity cop can be regulated constantly.

For quick adaptation, the coupling members 3 and/or lever arms 4 are tobe formed in accordance with the disclosure in such a way that theirmean density is at least occasionally lower than the surrounding fluid.The inertia of the rotor 2, 3, 4 is thus reduced.

Due to the effect of the flow having the incident flow velocity {rightarrow over (v)} on the coupling members, as explained in greater detailbelow, a first torque acting on the rotor 2, 3, 4 is produced. Inaddition, a preferably changeable second torque in the form of aresistance, that is to say a braking torque, or in the form of anacceleration torque, can be applied to the rotor 2, 3, 4. A prerequisitefor a torque of this type is a support means (mooring system) integratedinto the machine. With low rotor inertia, the energy expenditure for anacceleration of the rotor by means of an acceleration torque reduces.Means for producing the second torque are arranged between the rotorbase 2 and the housing 7. Here, the housing 7 is preferably the statorof a direct-driven generator, and the rotor basis 2 is the rotorthereof, of which the bearing, windings, etc. are not illustrated. Inthis case, the second torque is determined by the generator torque.Alternatively, other drivetrain variants can also be provided however,in which the means for producing the second torque also comprise,besides a generator, a transmission and/or hydraulic components, such aspumps. A suitable brake may also be provided, either in addition orexclusively.

A phase angle α, of which the value can be influenced by the adjustmentof the first and/or of the second torque, exists between the rotororientation, which is illustrated by a lower dashed line and which runsthrough the rotor axis and through the middle of the two adjustmentdevices 5, and the direction of the orbital flow, which is illustratedby an upper dashed line and which runs through one of the velocityarrows {right arrow over (v)}. Here, a phase angle from −45° to 45°,preferably from −25° to 25°, and more preferably from −15° to 15°, forproducing the first torque appears to be particularly advantageous,since in this case the orbital flow and the incident flow are orientedso as to be largely perpendicular to one another due to the inherentrotation v_(R) (see FIG. 5), which leads to a maximization of the rotortorque. If the required synchronicity is maintained, α≈const., whereinan oscillation about a mean value of α is understood to be synchronous.The coupling elements are illustrated in FIGS. 4 and 5 merely by way ofexample for definition of the different machine parameters. Duringoperation, the angle of attack of the two coupling members 3 maytherefore also be opposite compared to that shown in the illustration.The left coupling member in FIG. 4 would then be adjusted inwardly, andthe right coupling member in FIG. 4 would then be adjusted outwardly.Here, in contrast to this schematic illustration with uncurved,symmetrical profiles, the use of other profile geometries may also bepossible, which can additionally also still be adapted and/ortransformed with regard to the circular path.

Within the scope of the disclosure, {right arrow over (v)} and Ω forexample can be predetermined, such that a preliminary control of thefirst and/or second torque can be carried out accordingly. The firsttorque is influenced substantially via the angles of attack α_(P,1) andα_(P,2) and also via the phase angle α between rotation ω and orbitalflow Ω and the resultant incident flow velocity, and the second torqueis influenced substantially via the torque tapped by the generator,which for example can be influenced by a default setting of theexcitation current of the rotor. When determining monochromatic wavestates, the mean density of the components of the rotor 2, 3, 4 can beincreased, or by contrast reduced when determining multichromatic wavestates. This can be achieved for example by pumping water into or outfrom coupling members 3 formed as hollow profiles.

In FIG. 5, the occurring incident flow conditions and forces at thecoupling elements, which lead to a rotor torque, are illustratedschematically. Here, it is assumed in a simplified manner that the flowis uniformly pronounced over the entire rotor cross section and has aconstant value and a constant direction. In particular for rotors havinglarge radial extents, it may be however, precisely in the case of theillustrated horizontal orientation of the lever arms 4, that the variouscoupling members 3 of the rotor 2, 3, 4 are located at differentpositions relative to the wave, which leads to an incident flowdirection that differs locally. This can be compensated for however withthe aid for example of an individual adjustment of the respective angleof attack α_(P,i).

On the coupling members illustrated to the left in FIG. 5, the localincident flows are represented by the orbital flow v_(W,1) and by theinherent rotation v_(T,1), the incident flow velocity v_(R,1) resultingas a vector sum from these two incident flows, and also by the angle ofincidence α₁. The lift and resistance forces F_(A,1) and F_(R,1) atthese coupling members, which are dependent on the value of the incidentflow velocity v_(T,1) and also on the angle of incidence α₁ andtherefore also on the angle of attack α_(P,1) and are orientedperpendicular or parallel to the direction of v_(R,1), are additionallyderived. With regard to the coupling member illustrated to the right inFIG. 5, only some of these parameters are specified and provided withthe index 2 for reasons of clarity.

For the illustrated case, a rotor torque in an anti-clockwise directionis produced as a result of the two lift forces F_(A,1) and F_(A,2), anda smaller rotor torque in the opposite direction (that is to say in theclockwise direction) is produced as a result of the two resistanceforces F_(W,1) and F_(W,1) (not illustrated). The sum of both rotortorques leads to a rotation of the rotor 2, 3, 4, of which the velocitycan be adjusted by means of the adjustable second torque. As can beseen, a significant proportion of the forces acts radially and should besupported by a suitable restraint system.

If a synchronicity with Δ≈const. is achieved, it can thus be seendirectly from FIG. 5 that, for monochromatic cases, in which the valueof the flow v_(W,1) or v_(W,2) and the angular velocity Ω remain largelyconstant (under consideration of the deep water and shallow watereffects explained previously), the incident flow conditions of the twocoupling members 3 do not change via the rotation of the rotor. Thismeans that, with constant angles of attack α_(P,1) and α_(P,2), alargely constant rotor torque is produced, which can be tapped with aconstant second torque of a corresponding generator. By contrast, in thecase of multichromatic waves, changes occur, which can be taken intoconsideration by adapting the angles of attack and/or the second torque.An adaptation of ω₁ is achieved in a particularly advantageous manner bymeans of a low inertia of the rotor.

What is claimed is:
 1. A wave energy converter comprising: a rotorconfigured to convert a wave movement of a body of water, in which thewave energy converter is disposed, into a rotational movement of therotor, wherein the rotor includes at least one rotating component, andwherein a mean density of the at least one rotating component of therotor is adjustable and/or is lower than a mean fluid density of a fluidsurrounding the rotor in the body of water.
 2. The wave energy converteraccording to claim 1, wherein the at least one rotating componentincludes at least one of a rotor base, a lever arm, and at least onecoupling member of the rotor that couples with the wave movement.
 3. Thewave energy converter according to claim 1, wherein the at least onerotating component includes two components of the rotor arrangedsubstantially symmetrically about an axis of rotation of the rotor. 4.The wave energy converter according to claim 3, further comprising: acompensation mechanism configured to compensate for differences in meandensities of the at least two components of the rotor distributeduniformly about an axis of rotation of the rotor.
 5. The wave energyconverter according to claim 1, wherein the at least one rotatingcomponent of the rotor is formed in shell construction.
 6. The waveenergy converter according to claim 5, wherein the at least onecomponent formed in shell construction includes at least one of (i) ashell formed from at least one of a composite material and a textilematerial and (ii) at least one of reinforcing struts and a foam fillingin an interior of the at least one component formed in shellconstruction.
 7. The wave energy converter according to claim 5, furthercomprising: a pumping device configured to pump a fluid into or out fromthe at least one component formed in shell construction so as to adjusta mean density of said at least one component formed in shellconstruction.
 8. The wave energy converter according to claim 7, whereinthe pumping device, in order to pump the fluid into or out from the atleast one component formed in shell construction, includes at least onefilter device configured to filter at least the fluid pumped into the atleast one component formed in shell construction.
 9. The wave energyconverter according to claim 7, further comprising: at least one furthercomponent, wherein the pumping device, in order to pump the fluid intoor out from the at least one component formed in shell construction, isconfigured to pump the fluid between the at least one component formedin shell construction and the at least one further component.
 10. Amethod for operating a wave energy converter, which has a rotor havingat least one rotating component and is configured to convert a wavemovement of a body of water, in which the wave energy converter isdisposed, into a rotational movement of the rotor, said methodcomprising: establishing and/or predicting at least one wave state inthe body of water in which the wave energy converter is disposed; andadjusting, on the basis of the established and/or predicted wave state,a mean density of the at least one rotating component of the rotor ofthe wave energy converter.
 11. The method according to claim 10, furthercomprising: increasing the mean density of the at least one rotatingcomponent of the rotor of the wave energy converter if the at least onewave state established and/or predicted is monochromatic; and/orreducing the mean density of the at least one rotating component of therotor of the wave energy converter if the at least one wave stateestablished and/or predicted is polychromatic.
 12. The method accordingto claim 11, further comprising: operating the rotor with a mean speedof rotation largely synchronously with respect to a water movement inthe body of water by at least one of braking and accelerating the rotor.13. A control device for a wave energy converter, which has a rotor withat least one rotating component and is configured to convert a wavemovement of a body of water, in which the wave energy converter isdisposed, into a rotational movement of the rotor, said control devicebeing configured to (i) establish and/or predict at least one wave statein the body of water in which the wave energy converter is disposed, and(ii) adjust, on the basis of the established and/or predicted wavestate, a mean density of the at least one rotating component of therotor of the wave energy converter.
 14. The wave energy converteraccording to claim 5, wherein the at least one rotating component of therotor is at least partly hollow.
 15. The wave energy converter accordingto claim 7, wherein the at least one further component is a stationarycomponent.